Its color ranges from yellow to a deep orange or brown. It is also the term given to the colors created by this pigment, particularly a pale brownish-yellow. A kind of ochre with a high concentration of hematite, or dehydrated iron oxide, has a reddish hue and is known as "red ochre" (or, in some dialects, ruddle). Other names for this mineral are aragonite, chrysotile, copihaita, gypse, hilomeles, jasper, magnesian limestone, marble, mica, pyrope, sapphire, schist, selenite, serpentine, stromatolite, tremolite.
Ochre was one of the first minerals to be used by man. It was used as a paint and ink ingredient, and also as a dye for fabrics and leather. Ochre contains iron oxides which can turn blue under certain conditions. When heated to about 500° C (930° F), ochre will release its colorants: protoporphyrin IX and bilirubin. These are the chemicals that give blood its red color.
In medicine, ochre has been used since ancient times as a counter-agent to white powder (such as arsenic compounds or lead carbonate). It was also used as an eye wash to treat infections and inflammations, and as a mouthwash to prevent tooth decay.
Ochre is still used in modern medicine.
The most often used pigments are produced from ochre, and their colors range from yellow to brown to red. Ochre is mostly made up of yellowish limonite, which is made up of hydrated iron (III) oxide-hydroxides combined with clay and sand. The deeper the color, the more mineral content it has.
The amount of iron in ochre determines its color: redder shades have more iron, while yellower ones have less. For example, Indian ochre has a green color because it contains little iron, while Spanish ochre is very red due to the presence of hematite iron (II) oxide.
Besides iron, other elements can also affect the color of ochre. For example, manganese gives ochre a violet color, while zinc produces gray tones.
Finally, oxygen can change the color of ochre as well. For example, when carbon is added to black ochre, it turns pink or reddish purple. Conversely, when oxygen is taken away from white ochre, it becomes brown.
In conclusion, the color of ochre is determined by two factors: the type of iron present and the amount of each element involved. Different types of ochres will always yield colors that lie on a spectrum between yellow and brown.
Limonite, the principal color-giving component of natural yellow ochre (ocher, yellow earth), is a combination of numerous iron-containing minerals, including goethite, akageneite, lepidocrocite, and jarosite, with goethite (iron oxide hydroxide a-FeOOH) being the predominant component. The source material for most commercial yellow ochers is either natural or synthetic.
Limonite can also be produced by chemical treatment of brown coal or black shale. This type of ocher contains significant amounts of chromium oxides that give it its orange-yellow color. It can also contain small quantities of other elements such as arsenic, antimony, and bismuth. Chromium-rich limonites are more red than yellow; those containing lesser amounts of chromium are yellow to light brown in color.
Natural yellow ochre consists primarily of magnetite (iron oxide), but also includes some hematite (iron oxide) and goethite. The main source of color is usually yellow gold clay, which is very rich in phyllosilicate minerals that give it its golden hue. Other sources include red sandstone, green sand, and yellow jasper. Synthetic ochers are made by treating ferrous metal with an acid solution of hypochlorous or hydrochloric acid. The resulting product is filtered and washed with water to remove any excess acid. Neutralized wastewater is often discharged into lakes or rivers rather than treated for disposal.
Yellow ochre is a natural earth pigment composed primarily of clay that has been tinted by iron oxides. Under moderate heat, yellowish-red hues are formed; however, the stronger the heat, the more rich and saturated the color is produced. Yellow ochre has been used for centuries and may be safely blended with other colours. It can be used in oil paints, acrylics, watercolors, and tempera paintings.
There are two main types of yellow ochre: raw and processed. Raw yellow ochre is the unrefined form of the mineral, which means that it still contains some small particles of clay and other substances with which it was originally mixed. These particles affect how well it will mix with other colors and can also affect how much you need per painting. The more refined raw ochre is available in market and it tends to be very pale in color. This type of ochre has been through a process called "bleaching" which removes most of the brown and red tones from the mineral and leaves only its yellow component.
Processed yellow ochre is manufactured by heating raw ochre in large tanks filled with water until all organic material has evaporated. The remaining material is cooled, washed, and ground up into a powder. This process eliminates most of the small particles that could cause problems when mixing with other colors.
Heating raw ochre causes any iron oxide present in the stone to oxidize or turn red.
What exactly is white ochre? Ochre is defined as an earth pigment containing at least 20% iron oxide and significant quantities of aluminum silicate, calcium carbonate, silicon dioxide, or other minerals. Any quantity of iron oxide would give the earth pigment a yellow colour. Natural Pigments Technical Director George O'Hanlon says that while there are many varieties of ochres, they can be divided into two main groups: brown and yellow-brown. White ochre is actually a mixture of red and yellow ochres, which are both derived from rocks composed mainly of hydrated iron oxides.
In painting, white ochre is used to indicate snow, ice, or cold conditions. The ancient Egyptians used it for body paint and to decorate coffins. It was also popular with Native Americans who applied it to their bodies during ritual ceremonies. In Europe, it was often used by monks as a way to color-code their cells.
There are several types of white ochre used in art materials. They vary significantly in color and texture. The most common type is called French ochre. This is a soft powder that can be used as a filler or glaze for paintings. It comes in various shades of orange and red. Chinese white ochre is similar to French ochre but slightly darker in color. It's used in the same ways by Chinese artists.
Indian white ochre is available in three grades.